This invention relates to a method for examining a surface of a sample, the state near the surface, and in particular the thickness of its machining damaged layer by means of a focused ultrasound beam and to a device for realizing the method.
Recently it has become possible to generate and detect sonic waves of ultra high frequency up to 1 GHz and thus to realize a sonic wavelength of about 1 .mu.m in water. As the result, it is disclosed in U.S. Pat. No. 4,028,933 to be able to obtain an ultrasound imaging apparatus having a high resolving power. That is, a high resolving power up to 1 .mu.m can be realized by forming a focused ultrasound beam by means of a concave lens.
By inserting a sample to be studied in the beam, detecting ultrasound reflected by the sample, and displaying the intensity of signals obtained while moving mechanically the sample two-dimensionally on the screen of a cathode ray tube by using them as the brightness signal, it is possible to observe the microstructure of the sample in an enlarged scale.
Further it is desclosed in Appl. Phys. Lett. 31 (12), Dec. 15, 1977, pp. 791-793 that elastic characteristics, the presence of scratches, etc. on the surface of the sample can be known by means of such a focused ultrasound beam. According to this article, characteristics of the surface of the sample can be estimated on the basis of a curve (called V(z) curve) obtained by plotting the intensity of reflected ultrasound, detected while varying the distance between the sample and the lens.
On the other hand, it is desired to establish a method for evaluating damages in very fragile materials such as ceramics, because the strength of such materials depends strongly on defects existing within the materials or on micro defects produced in the surface portion of the sample during machining.
However it has been considered that it is difficult to evaluate the degree of the defects causing deterioration in strength, i.e. the thickness of the machining damaged layer with a high efficiency in a nondestructive manner.